The present invention relates to methods and apparatus for providing digital communications between a plurality of independently operating units. In particular, such interconnections employ tag or control lines operatively associated with data bus lines.
In data processing installations, host computers have input-output connections to a plurality of control units or directors which in turn are connected to a hierarchy of input-output devices. Such interconnections can be multi-pathed and highly flexible for providing a large diversity of functions with varying capacities. The connections from the host computers to the input-output control units or directors have been developed to be highly reliable, highly flexible and to have high data rates. Such interconnections are expensive and the expense is justified as between a host computer and its group of input-output directors. An example of such input-output connection is the input-output connections provided by the IBM 370 series computers, manufactured by International Business Machines Corporation, Armonk, New York. That set of interconnections provide for a plurality of tag lines which define, in a so-called "handshaking manner", the operational interrelationship between a host computer and a selected director. In general, a single tag line will carry a single tag signal such as OPERATION OUT, SELECT OUT, SERVICE OUT, DATA OUT and the like.
A similar type of interconnection is required between each of the input-output or peripheral directors/control units and their respective connected input/output devices; such as printers, record disk apparatus, communication modems, tape units and the like. Generally, the cost justification for the director to device unit connection is not as high as for the host-to-director interconnection. Accordingly, it has been the practice in the connections from the device to the control unit to simplify the connection such that a smaller cable can be used with fewer controls. Generally, the operational interrelationship between a director and its connected devices is much simpler than between the host and the directors; that is, the communications or interconnection protocols can be simpler because of the simplified relationship. For example, in the connection cable between a host computer and a director, there can be 96 wires (physically divided into two cables/connectors) of which 40 (20 signals with ground wires) are used for tag lines and 18 are used for data bus lines, 9 wire pairs for a bus out and 9 wire pairs for a bus in. The connection from a director to a device on the other hand can be achieved by a mere 24 lines. Such a connection is exemplified by the teaching in U.S. Pat. No. 3,654,617.
With the advent of large scale integration, microprocessors, programmable logic arrays on a chip, and the like, the device control and the director control have become much more complex and expensive. For example, a device may be microprocessor controlled with the control program being loaded into a microprocessor random access control unit from a tape unit or disk. In a similar manner, a director can have one or more microprocessors with a plurality of programmable logic arrays, all of which have volatile control memories and are loaded as described for the device microprocessor. Such microprocessor PLA's (programmable logic arrays) and the like perform the logic of operation for the directors and device. Such logic of operation is now becoming very, very complex requiring a greater complexity into interrelationship between a director and a device. Yet for cost control reasons, it is desired not to create a larger cable for accommodating this increased subsystem (director-device configuration) complexity. Accordingly, an improved method and apparatus must be provided for accommodating such increased complexity without substantially adding to the interconnection costs.
Further, error conditions and error recovery become much more feasible with program logic than with fixed logic. Also, the probability of error in a more complex subsystem is higher. Therefore, the ability to automatically detect an error condition, get error status information and recover from error conditions over the same 24 lines or less becomes an important aspect of subsystem capabilities.
The resulting operational interrelationships between a host computer and its director also become more complex. Accordingly, it is also desired to enhance the operational interrelationship between the host and the director by creating a greater interconnection capability without substantial added cost.